The present invention is directed to a control system for magnetic head positioners (hereinafter, simply referred to as positioners) disposed in a multi-positioner magnetic disk storage apparatus. In particular, the present invention is directed to a control system for preventing a magnetic head from becoming off-track during a read/write operation due to a mechanical vibration caused by a simultaneous seek operation by other magnetic heads. Furthermore, the present invention is directed to a system for compensating a driving signal for a positioner during a read/write operation such that an adverse effect due to a mechanical vibration caused by a seek operation by other positioners is cancelled. In order to do this, the mechanical vibration is sensed with an acceleration sensor disposed on the surface of a housing of the relevant magnetic storage apparatus.
The requirement for increased storage capacity of a magnetic disk has substantially increased to a track density of approximately 1200 tracks/inch, that is, a track pitch of 20 micrometers. During a read/write operation, the magnetic head must be accurately positioned on a track formed on the magnetic disk. Although the accuracy is on the order of sub-microns even greater accuracy is required.
A magnetic disk storage apparatus usually includes a plurality of magnetic disks assembled around a rotating spindle forming a magnetic disk assembly. Due to the vast quantity of information stored in the magnetic assembly, concurrent access occurs where two or more positioners are being accessed simultaneously. As a countermeasure against concurrent access a multi-positioner magnetic disk storage apparatus having plural positioners which operate independently from each other has been developed.
A positioner performs two kinds of operations; a seek operation which may be referred to as a coarse access operation, and a track follow-up operation, which may be referred to as a fine access operation. During the seek operation the positioner operates to transfer the relevant magnetic head supported by the positioner from the present track to a target track under control of a central controller, which may be, for example, a computer. The track following operation starts when the magnetic head reaches a position extremely near the target track. The positioner is driven such that the magnetic head can accurately follow the target track for a read/write operation. When the magnetic head is in this state it is referred to as being "on track". Naturally, in a multi-positioner magnetic disk apparatus, there occurs a situation where one of the positioners is performing a follow-up operation and at the same time another one of the positioners is performing a seek operation. The positioner performing the seek operation is driven with a high accelerating force during the seek operation so that it is subject to a strong reaction force. The reaction force causes a strong mechanical vibration of the storage apparatus, causing the magnetic head supported by the positioner performing the read/write operation to be off-track. Such mechanical interference between the positioners has been a serious, inevitable problem inherent to multi-positioner magnetic disk storage apparatus. Apparently, when all the magnetic heads are performing read/write operations, such problem does not occur.
There are two types of positioners used in a magnetic disk storage apparatus; a linear type positioner and a rotary type positioner. In a linear type positioner the magnetic heads are driven in a radial direction towards the center of a rotating magnetic disk assembly. In a rotary-type positioner the magnetic heads are driven along an arc across a magnetic disk surface, and the positioner rotates in a clockwise and counterclockwise direction around a rotating axis of the positioner. In a multi-positioner magnetic disk storage apparatus having linear-type positioners, the above-described mechanical interference is more serious in comparison with a multi-positioner magnetic disk storage apparatus having rotary-type positioners. This is because the reaction force of the linear-type positioner is much higher than that of the rotary-type positioner. Particularly, when linear-type positioners are stacked one above another and the moving directions of the magnetic heads are the same, the full reaction force adversely affects the positioners during a read/write operation.
FIG. 1 is a schematic diagram of a prior art control system of a multi-positioner magnetic disk storage apparatus. In the magnetic disk storage apparatus a plurality of magnetic disks 11 (shown as three disks) as recording media are stacked in parallel with fixed spaces therebetween, around a spindle 12 which is rotated by a motor (not shown), forming a magnetic disk assembly 10. Two pairs of read/write heads 13a and 13b, and servo-heads 14a and 14b are supported by positioners 20a and 20b, respectively, through head arms 15a and 15b. Both positioners 20a and 20b are driven independently under control of respective control circuits (only one series of circuits is shown in FIG. 1). The head arms 15a and 15b are fixed to carriages 19a and 19b which are driven by voice coil motors 18a and 18b (hereinafter referred to as VCMs) along respective seek directions or access directions. The VCMs 18a and 18b include drive coils 16a and 16b (not shown in FIG. 1), magnets 17a and 17b (not shown in FIG. 1) and magnetic circuits including yokes (not shown). The magnetic heads 13a, 13b, 14a and 14b, the head arms 15a and 15b, and the carriages 19a and 19b form moving bodies 19a and 9b, respectively. Usually, the magnetic 1 head positioners 20a and 20b and magnetic disk assembly 10 are installed within a dust proof housing 21.
The control circuits of the positioners are now described. A position signal generator 9 receives servo-signals SVS sent from the servo head 14a and outputs position signals PS. A velocity controller 2 receives the position signal PS and generates velocity error signals .DELTA.V. A magnetic head positioner controller 4, in response to access command signals sent from a central computer (CPU) I, controls the velocity controller 2 to output the velocity error signals .DELTA.V for controlling coil current of the VCM 18a, which controls the rotating speed of the VCM 18a. In addition, when the magnetic head 14a is in the vicinity of the target track the magnetic head positioner controller 4 issues a coarse/fine switching signal MS to activate a switch 5.
A positioning controller 3 receives and amplifies the position signal PS, performs integral processing and differential processing (P-I-D processing), and adds a detection current i output from a power amplifier 50 to the processed position signal PS. The resultant current is then filtered through a low-pass filter (not shown), thus generating a position error signal .DELTA.P. The switch 5 operates such that the velocity error signal .DELTA.V is sent to the power amplifier 50 as a coarse access signal, and the position error signal .DELTA.P is sent to the power amplifier 50 as a fine access signal. Both the position error signal .DELTA.P and velocity error signal .DELTA.V are amplified by the power amplifier 50 and input to the coil of the VCM 18a.
FIGS. 2(a)-2(c) are timing diagrams of the switching operation of the switch 5. As will be described later, these timing diagrams are applicable to magnetic disk storage apparatus according to the present invention. The ordinates of FIGS. 2(a), 2(b) and 2(c) show the distance between a target track (or a target cylinder) and the present position of the relevant magnetic head, the signal height of a coarse/fine switching signal MS, and the signal height of a seek end signal, respectively. The abscissa is with respect to time.
The operation of the magnetic head positioner control system of the prior art magnetic disk storage apparatus shown in FIG. I is described briefly with reference to FIG. 2. The seek operation is performed as follows. The magnetic head positioner controller 4, after receiving an access command signal from the CPU 1, calculates the distance, e.g., the number of tracks, between the target track and the present position of the magnetic head 14, and generates a criterion velocity Vc by referring to a velocity-time function having a predetermined storage pattern having, for example, a trapezoidal shape. The position signal generator 9 generates a position signal PS based on the servo-signal SVS supplied from the servo-head 14. A real velocity Vr is generated in the velocity controller 2 by processing the position signal PS output from the position signal generator 9. The velocity error signal .DELTA.V is generated in the velocity controller 2 by comparing the real velocity signal Vr with the criterion velocity Vc, and the comparison signal is output to a terminal a of the switch 5. At this stage, the terminal a is in an ON state due to the absence of the coarse/fine switching signal MS. Thus, the velocity error signal .DELTA.V is input to the power amplifier 50 where it is amplified and applied to the coil of the VCM 18. As a result, the positioner 20 is driven with a velocity which varies following the trapezoidal time pattern. That is, the positioner 20 is subject to acceleration, non-acceleration and deceleration in that order. Substantially strong reaction forces are caused in the access direction in which the moving member 119a of the positioner 20a is driven.
When the magnetic head positioner controller 4 detects that the servo-head 14 has reached the vicinity of the target track within a predetermined tolerance at time T.sub.1 as indicated in FIG. 2(a), then a fine/coarse switching signal MS is issued from the controller 4 to the switch 5, turning the terminal b ON and the terminal a OFF. As a result, application of the velocity error signal .DELTA.V to the power amplifier 50 is terminated and the position error signal .DELTA.P output from the positioning controller 3 is applied to the amplifier 50, as indicated in FIG. 2(b). Thus, the positioner 20 performs a fine access operation or track following operation.
The position signal PS generated by the servo-head 14 in accordance with the servo-signal SVS is amplified by the power amplifier 50 and applied to the coil of the VCM 18a . Thus, the off-track of the servo-head 14 is corrected. After a settling time t.sub.s elapses from the time T.sub.1, a seek end signal SE is issued from the magnetic head positioner controller 4 at time T.sub.2 as indicated in FIG. 2(c). This allows a read/write operation by the magnetic heads 13.
The capability of the above-described prior art control system for preventing the magnetic head from becoming offtrack, e.g., for preventing a servo-gain in the system, is limited to within a predetermined value. Furthermore, the limit is narrowed by the mechanical interference between a positioner during a seek operation and a positioner during a read/write operation, causing mechanical vibration of the apparatus due to the reaction force of the positioner during the seek operation. This mechanical interference is inevitable in a multi-positioner magnetic disk storage apparatus. The adverse effect becomes more serious when the resonant frequency of the mechanical vibration is low.
There are several proposals for overcoming the above problems caused by mechanical vibration. In order to minimize the shock of the reaction force caused by a positioner during a seek operation, an elastic suspension mechanical system for the positioners was proposed by the inventors of the present invention wherein the fixed portions of positioners such as the yoke, coils, and magnets, are suspended against a housing of the magnetic disk storage apparatus by spring plates. Damping elements are disposed between the housing and the fixed portions of the positioners. By this mechanism, the mechanical shock is reduced a certain degree, but a mechanical resonant vibration of 100 to 150 Hz is caused. Thus, the proposals for preventing the magnetic heads from becoming off-track is still unsatisfactory.
Japanese Provisional Published Laid Open Patent Application No. 60-121578 published on June 29, 1985, and No. 61-170967 published on Aug. 1, 1986, both to Moriya et al., disclose an optical disk storage apparatus. In the apparatus in the Moriya et al. references, an acceleration sensor secured to a housing of the apparatus detects mechanical vibration caused by an external force subjected to the apparatus. The optical storage apparatus of the Moriya et al. references is a single positioner type. Mechanical interference is not a problem, and is therefore, not disclosed by the Moriva et al. reference. A low-pass filter having a band width ranging from 1 Hz to 1000 Hz is used to filter signals output from the acceleration sensor to eliminate high frequency noise contained in the signal.
Therefore, a magnetic disk storage apparatus capable of overcoming the mechanical interference problem between plural positioners during operation is needed.